The invention relates to a device for controlling an internal combustion engine.
In the field of motor vehicles it is important to be able to have precise control over the combustion phase of the thermodynamic cycle of these engines. To do that, it is commonplace to measure the pressure in the combustion chamber and, on the basis of this measurement, to determine a certain number of characteristic parameters, particularly parameters that can be used for feedback control of the engine. These parameters are, for example, the combustion start time, the power of acoustic emissions from the engine, or the power supplied by the engine. However, the pressure signal obtained by measurement in a combustion chamber cannot be used without prior processing because it exhibits measurement noise in the form of parasitic fluctuations. An example of such a raw pressure signal p(t) measured in a combustion chamber of a car engine is depicted as a function of time in FIG. 1A, FIG. 1B being an enlarged detail of the signal depicted in FIG. 1A, this detail corresponding to the start of combustion. The fluctuations present on such a signal do not correspond to variations in the pressure in the combustion chamber but are due to parasitic mechanical fluctuations. The measured pressure signal must therefore be filtered before it can be used to determine parameters that can be used for feedback control.
Simple solutions involving linear filtering allow the undesirable fluctuating components in the pressure signal to be reduced. However, this type of filtering has the disadvantage of also attenuating the frequency component, located at around 10 kHz, which corresponds to a sharp increase in the pressure at the start of combustion (in FIG. 1A or 1B, this sharp increase occurs at around t=0.015). Precise detection of the start of combustion is therefore in particular not possible with such a method.
Other known solutions rely on the use of wavelet transforms. Hence, patent application US 2003/0 145 829 A1 describes a method allowing the start of combustion to be detected in a combustion chamber from a measurement of the pressure obtaining in this chamber. The pressure signal is filtered by applying a wavelet transform. The start of combustion is detected by analyzing the wavelet coefficients thus obtained, in that it manifests itself in an abrupt jump in the absolute value of the wavelet coefficients.
Another source, patent application EP 1 209 458 A1 describes a method for determining the level of noise relative to the combustion noise of an internal combustion engine. The measured pressure signal is also filtered by wavelet transforms. The energy of the starting time-based signal may, on the basis of Parseval's theorem, be estimated from the wavelet coefficients obtained and it is possible from this to deduce the noise level. This noise level can be used as a feedback control parameter for an engine combustion control module.
However, these methods provide only partial solutions to the problem that is that of determining the parameters needed for controlling combustion. These methods are not, in particular, able to calculate directly and accurately the apparent energy release, as defined by the following relationship:
                                                        ⅆ              Q                                      ⅆ              t                                ⁢                      |            t                          =                                            1                              γ                -                1                                      ⁢                          V              ⁡                              (                t                )                                      ⁢                                          ⅆ                P                                            ⅆ                t                                              ⁢                      |            t                    ⁢                                    +                              γ                                  γ                  -                  1                                                      ⁢                          P              ⁡                              (                t                )                                      ⁢                                          ⅆ                V                                            ⅆ                t                                              ⁢                      |            t                                              (        1        )            where γ is the ratio of the specific heat capacities of the combustion gases (the ratio of the specific heat capacity at constant pressure to the specific heat capacity at constant volume), V(t) is the volume of the combustion chamber, P(t) is the pressure in the combustion chamber and t is the time. To this end, it is therefore necessary, the volume of the combustion chamber being known, to be able to determine the instantaneous pressure actually obtaining in the combustion chamber and the derivative of this pressure with respect to time and therefore to greatly reduce or eliminate the parasitic fluctuations contained in the measured pressure signal usually available.
French patent application FR 04 07060 itself describes a method for processing a measured pressure signal from a combustion chamber of an internal combustion engine using a wavelet-based filtering technique and nonlinear filtering functions and able to yield a filtered pressure signal which is suitable for determining parameters such as the apparent energy release or the combustion start time. This method does, however, entail an embodiment based on a digital processor of the DSP type or based on an ASIC (application-specific integrated circuit) and is therefore a relatively expensive embodiment.